1. Field of the Invention
The present invention relates to a light-supplying optical device used in an image reading system that electrically reads image information of an original document, and, in particular it relates to a light-supplying optical device capable of suppressing the line bow phenomenon produced by a mirror, such as for example, a toric mirror that collects light from the light source into a linear form.
2. Description of Related Art
FIGS. 7 through 10 show the structure of one type of conventional image reading system. Referring to FIGS. 7 and 8, the image reading system includes an illuminating unit 3 that conducts light supplied from a light source 1 to the surface of an original document 2. The image reading system also includes a carriage 4 that supports and moves the original document 2, and a projection unit 6 that forms the light that passes through the original document 2 into an image on a linear sensor 5 (e.g., a CCD:Charge-Coupled-Device).
The illuminating unit 3 includes a plate-like base member 7 on top of which are attached a light source 1 that emits light radially, a first mirror 8 and a second mirror 9 that change the direction of the light so that it is formed into a line on the surface of the original document 2. An illuminating unit lid 11 containing a slit 10 through which light can pass is fixed to the base member 7 by means of a hook-shaped fastener 11a so as to cover the light source 1, the first mirror 8 and the second mirror 9. Light from the light source 1 is collected by the first mirror 8 so as to form a line on the surface of the original document, and is bent (i.e., reflected) in the direction of the original document, i.e. in the vertical direction, by the second mirror 9.
Accordingly, light from the second mirror 9 is formed into the shape of a long, thin rectangle (i.e., an area of light extending in a primary scan direction) close to where it passes through the slit 10 in the lid 11. Because the slit 10 provided in the lid 11 should be the size needed for the passage of light, it is in the shape of a rectangle, slightly larger than the rectangular shape of the light. In addition, a plate 11b is bent inward on part of the slit 10 so as to prevent external light from the opening 12 provided in the front of the base member 7 from reaching the light source 1 by passing through the slit 10.
The upper carriage portion 4a and lower carriage portion 4b that support the original document 2 are guided by two parallel guide bars 13 so as to be capable of moving to the left and right as shown by arrow A--A (in what is known as the secondary scan direction). A rack unit, not shown in the drawing, is provided on part of the upper carriage 4a, and is engaged with a pinion gear. The carriage 4 moves back and forth by means of the pinion gear engaging with the carriage rack, the pinion gear being driven by a stepping motor or similar means.
The projection unit 6 includes a third mirror 14, a lens 15, a CCD 5 and a projection unit lid 17 that covers the entire unit and is provided with a slit 16 that allows light passing through the original document to enter the projection unit 6. In addition, a plate 17a is bent inward from the slit 16 in the projection unit lid 17 as was the case with the illuminating unit lid 11, the plate 17a preventing external light from the opening 12 from entering the projection unit 6 through the slit 16. The illuminating unit lid 11 and the projection unit lid 17 both have surfaces that are black and have been deglossed so that the lids themselves do not reflect external light.
FIGS. 9 and 10 show the composition of the light source of such a conventional device. The light source 1 has six blue LEDs 21, which function as a first light producing means, and four green LEDs 22 and two red LEDs 23, which function as a second light producing means, each of the LEDs 21, 22 and 23 being mounted on a stem 24. The blue LEDs 21 are arranged in a single straight line, and the green LEDs 22 and red LEDs 23 are arranged in a parallel straight line. The optical axis of light produced by the LEDs 21, 22 and 23 and reflected by the surface of a mirror 25 provided in the light source 1 as a reflecting optical system is positioned so as to fall in a single plane.
The 12 LEDs 21, 22 and 23 are mounted on a stem 24 formed of electrically conductive materials in a plate shape, one electrode of each LED 21, 22 and 23 being connected to the stem 24. The other electrode of each of the LEDs 21, 22 and 23 is connected via a wire 28 to an electrode 27 mounted on the stem 24 by means of an insulating material 26. A conical reflector 24a that reflects light produced in the sideways direction and directs it upward is formed in the stem 24 around each of the LEDs 21, 22 and 23. Light reflected by each of the reflectors 24a is reflected by the reflecting mirror 25 and directed toward the front (the right in FIGS. 9 and 10), and is further collected in a linear form on the surface of the original document 2 by mirrors 8 and 9.
The light produced by the blue LEDs 21 is reflected by a blue reflecting film 25a formed on a first surface of the reflecting mirror 25, while light from the green LEDs 22 and red LEDs 23 is reflected by a wholly reflecting film 25b, formed on a second surface of the reflecting mirror 25. As a result, when viewed from the front, it appears that all three colors of light are emitted from the same position. In addition, switching between the three colors blue, green and red is controlled electrically, making it possible to read the original document 2 at high speed. A 45.degree. prism 29 also is provided on the light-emitting surface of the reflecting mirror 25.
A toric mirror is used as the first mirror 8. The toric mirror 8 is comprised of a curved surface having curvature in both the lengthwise direction (i.e., the horizontal direction as shown in FIG. 8) and in a direction perpendicular to the lengthwise direction (i.e., the vertical direction as shown in FIG. 7). The arrangement is such that light produced by the light source 1 illuminates a width of one line on the document by means of the curved surface in the lengthwise direction. That is, the curvature R1 in the lengthwise direction causes the rectangle of light reflected by toric mirror 8 to have a certain length. The light image provided by light source 1 is formed into a linear image onto the surface of the original document by means of the curved surface in the direction perpendicular to the lengthwise direction. That is, the curvature R2 in the direction perpendicular to the lengthwise direction causes the rectangle of light reflected by toric mirror 8 to have a certain width, which usually is much less than the length.
Light emitted from the light source 1 is incident on the toric mirror 8 at an angle .THETA. relative to a line 18 normal to a central portion of the mirror 8. Accordingly, the light reflected by the toric mirror 8 is reflected at the angle .THETA. relative to the line 18. Such light is formed into an image on the surface of the original document 2 by means of the second mirror 9, which also is referred to as an optical path conversion mirror. By moving the carriage 4 that holds the original document 2 in the secondary scan direction indicated by arrows A--A, the CCD 5 successively reads image information from the entire surface of the original document (one line at a time).
However, with the described conventional image reading system, light emitted from the light source 1 is incident at the angle .THETA. from the line 18 normal to the toric mirror, while the toric mirror has curvatures R1 and R2 in the perpendicular directions as shown in FIGS. 11A and 11B, which are top and side views, respectively, of the light-supplying optical system in an unfolded state. Consequently, the angle of light reflection varies between the ends and the center of the toric mirror 8. As a result, the light source image 31 formed on the surface of the original document 2 is bow-shaped, as shown in FIG. 11C, thus creating the so-called line bow phenomenon. On the other hand, because the reading line 32 of the CCD 5 on the surface of the original document is a straight line, a problem arises in that lighting cannot be obtained that is uniform at both the center and at the ends of the reading line 32, as indicated in FIG. 11D, which is a graph showing the amount of light received by the CCD 5 relative to positions along the CCD 5.